1. Field of the Invention
This invention relates to solid state lighting devices. More particularly, it relates to solid state directional lighting devices having high luminous efficacy with high color rendering index.
2. Brief Description of the Related Art
It is well known that directional halogen lamps, like PAR lamps and MR lamps, or conventional ceiling spot lights, are very energy inefficient light sources; about 85-90% of the electricity they consume is released as heat rather than light. Accordingly, efforts have been ongoing to develop solid state directional warm white lighting devices to replace these inefficient warm white halogen lamps and spot lights in a wide variety of applications. Moreover, where solid state lighting devices are already being used, efforts are ongoing to provide improvement with respect to energy efficiency, color rendering index, luminous efficacy (lm/W), color temperature, and duration of service, especially for indoor applications.
Conventional solid state directional lighting devices usually utilize phosphor converted semiconductor warm white light emitting diodes (LEDs) in order to replace traditional directional halogen lighting devices which provide warm white color with correlated color temperature (CCT) of 2700K and 3000K. The phosphor converted warm white LED utilizes a blue LED with peak wavelength of 440 nm˜470 nm and a luminescent conversion layer directly deposited on top of an LED die containing a mixture of micro-particles phosphors, to produce a warm white light from the mixture of primary blue spectral light and excited green or yellow light with peak wavelength of 545 nm˜575 nm and excited reddish light with peak wavelength of 610 nm˜630 nm. However, conventional phosphor converted warm white LEDs have inadequate luminous efficacy due to multi-phosphors self-absorption loss, back-scattering loss of micro-particles and less human eye sensitivity of deep red light due to wide spectral width (full width of half maximum, FWHM) of reddish excitation light. They also exhibit phosphor degradation due to high LED case temperature. They also have a relatively low color rendering index. Conventional solid state directional lighting devices usually have color rendering indices (CRI/Ra) of only around 80, compared to CRI of 100 from incandescent light bulbs and halogen lamps. Further improvement of CRI to above 85, even above 90 is possible, but involves a deep red excitation light which in trade-off has 15-25% further loss of luminous efficacy.
Recently, efforts are ongoing to develop remote phosphor solutions which position conventional luminescent conversion phosphors in spaced relation away from direct contact with a semiconductor LED die to reduce phosphor degradation associated with elevated temperatures and to inhibit light loss resulting from back-scattering into the LED die. This conventional remote phosphor solution does not overcome the excessive light scattering problem due to its micro-particles size when used in directional lighting devices.
Thus there is a need for improved solid state directional lighting devices having high luminous efficacy without substantial phosphor degradation. There is a need as well for such devices having higher color rendering indices with less light scattering.
However, in view of the art considered as a whole at the time the present invention was made, it was not obvious to those of ordinary skill in the field of this invention how the shortcomings of the prior art could be overcome to provide a solid state directional lighting device having improved luminous efficacy, reduced phosphor degradation, increased color rendering index, and reduced light scattering.